This invention relates to a comparative colorimeter, and more specifically to an instrument which simultaneously compares the color (i.e., optical) densities of two liquid samples and designates the degree of difference. A colorimeter is useful for conducting:
(a) medical diagnostic tests based on enzyme immuno assays (EIA), e.g., the test for Leutenizing Hormone as an indication of ovulation in humans;
(b) medical diagnostic tests based on Enzyme Multiplied Immuno assay Technique (EMIT), e.g., tests for drugs of abuse such as barbituates;
(c) veterinary diagnostic tests based on either (a) or (b), e.g., progesterone level in dairy cows;
(d) environmental tests based on either (a) or (b), e.g., residual Chlorodane levels after extermination around residences; and
(e) determination of the concentration of certain chemicals based on absorption of a given wavelength of light, e.g., concentration of the herbicide Dynaseb.TM..
In the above-identified tests, it is generally not the absolute value of optical density of a solution which is of interest, but rather a comparison of a sample solution with a standard solution. Further, while all of these tests can be satisfactorily performed in the laboratory using a standard off-the-shelf spectrophotometric instrument, such instruments are delicate and expensive and are not adapted for use in the field. For example, a colorimeter is useful in the field to determine whether the residual level of a pesticide applied to a crop has declined sufficiently to allow safe entry of personnel into the field for harvesting operations.
A known colorimeter used in the field performs separate quantitative determinations of the optical densities of a sample solution and a standard solution. The human operator then computes the difference between the quantitative determinations. This device has several disadvantages. First, since the operator is generally interested in only the relative difference between the sample and standard, it is a waste of effort to make a quantitative determination for each of the sample and standard. Further, since the quantitative determinations of the sample and standard are made at different times, there exists a large potential for error if either of the solutions changes in optical density with respect to time or temperature.
It is an object of the present invention to provide a relatively low-cost comparative colorimeter for use in the field for comparing the optical densities of two solutions and indicating the relative degree of difference.
It is another object of this invention to provide a comparative colorimeter for simultaneously evaluating the optical densities of sample and standard solutions prepared at the same time in order to eliminate any error caused by changes in the optical densities of the sample and standard with respect to time.
It is a further object of this invention to provide an optical subsystem for a colorimeter which reduces the error caused by defects in the sample tube such as scratches, irregularities or striations in the tube wall, and lack of concentricity.
It is still another object of this invention to provide an electronic subsystem, including low-cost digital integrated circuits, for quantifying the difference between the optical densities of the sample and standard solutions.
It is a still further object of this invention to provide a correction circuit to compensate for variations in the threshold voltage of the digital IC components with respect to batch, time, temperature, and supply voltage.
It is still another object of this invention to provide a correction circuit to compensate for differences in the optical paths of the sample and standard solutions.